Imaging of moving objects (eg: organs in the chest and abdomen, the heart, moving joints) is of increasing clinical and medical research interest. A common solution to this problem is to acquire multiple sets of motion-frozen images, each of which is reasonably motion-free, but which taken together do not form part of a consistent representation of the subject. For diagnostic purposes, these motion-frozen components may be sufficient, but for quantitative analysis (eg: measuring structure volumes, or tissue function), it is necessary to generate a geometrically consistent set of data from all the motion-frozen images in order to obtain reliable measurements of the quantitative parameters. This applies, for example, to medical imaging modalities including MRI, CT, PET, SPECT and ultrasound. For MRI, the imaging might be from one of the many methods that require multiple correctly aligned measurements in order to derive the quantitative parameters e.g: diffusion imaging (DWI or DTI), functional MRI (BOLD fMRI), MRI relaxometry (e.g: T2* quantification from iron oxide contrast agents).
An especially challenging task is Foetal brain imaging by MRI which is attracting increasing interest because it offers excellent contrast and anatomical detail. However, unpredictable foetal motion has led to the widespread use of single shot techniques that can freeze foetal motion for individual slices. This provides high quality anatomical slices but these are generally inconsistent with one another. We previously developed a method, Snapshot images with Volume Reconstruction (SVR), for three dimensional (3D) high resolution and high SNR in-utero anatomical imaging of the foetal brain using dynamic scanning and image registration (see refs. 1-3). Fine structure of the in-utero foetal brain is clearly revealed for the first time at millimetric resolution from all three dimensions and substantial SNR improvement is realized by having many individually acquired slices contribute to each voxel in the reconstructed image.
An even greater challenge is in-utero Diffusion Weighted Imaging (DWI). It is highly sensitive in detecting certain brain diseases such as hypoxic-ischemia or periventricular leukomalacia (see refs. 4, 5) as well as for observing normal cerebral development. Preliminary trials (see ref. 6) have been performed in which for a few relatively thick slices, a b=0 (reference) together with 3 directions of diffusion weighted images were acquired within a maternal breath hold time, e.g. 20 seconds. These early experiments relied on the chance event of the foetus remaining still for all 4 sets of images so that apparent diffusion coefficients (ADC) could be calculated.
Diffusion tensor imaging (DTI) offers the potential for More information than DWI, and has been widely used in both adult (7) and neonatal studies(8, 9), particularly for tractography studies. However, DTI is even more challenging than DWI for foetal imaging, because it requires a b=0 image and at least 6 diffusion images that are sensitized in non-collinear directions for each slice studied. These extra images increase the minimum acquisition time so that the requirements for the maternal breath-hold become more onerous. Without a maternal breath-hold foetal motion combines with the mother's respiration to disrupt the spatial correspondence between component images required to calculate tensor properties. Moreover, the added diffusion sensitizing gradients make the diffusion weighted images highly sensitive to even small amount of motion. This results in data that is frequently corrupted by in-plane motion as well. Very recently, Bui et al (see ref. 10) reported diffusion tensor imaging with 6 non-colinear directions with in-plane resolution is 2.95 mm×1.09 mm, and 5 mm slice thickness. The whole sequence took 53 second and the mother was sedated. Besides the ADC value, they measured some Fractional Anisotropy (FA) values but without showing any FA maps.
The challenge of imaging subjects that cannot remain sufficiently still for conventional methods to be applicable is much more widespread than foetal imaging and includes subjects such as babies or children who may not be able to remain still and categories of patients who are unable to follow instructions or are subject to uncontrollable movements. Likewise the methods proposed are applicable to any other imaging protocol that requires multiple measurements to be combined. Example include functional Magnetic Resonance Imaging (fMRI), perfusion imaging, dynamic contrast imaging, magnetization transfer imaging, quantitative T1 or T2 imaging, temperature imaging and elastography as well as other modalities such as CT, PET and ultrasound.